Replies from some contacts are received in a separate group message thread with my address prefixed by a ", making replying impossible as it is an invalid address. Why?

This began happening after installing iOS7. It seems to be a little different than the basic messages bug that was fixed. I send a message to the person, they reply and it is put in a group message including my email like ( " asdf@softbank.ne.jp ). I then get a send failure if I try replying in the group message. I have to go back to the previously okay one-to-one message thread and reply, to which the reply from the other person once again goes to the group message. Really frustrated.